1. Field of the invention
The invention is generally related to a scanning tunnelling or atomic force microscope and more particularly to the manufacturing of tip and probe used in said scanning tunnelling or atomic force microscope.
2. Description of the Related Art
Scanning tunnelling microscopy (STM) and electrical atomic force microscopy (AFM) enables us today to carry out electrical and/or topographical measurements on modern semiconductor devices with nanometer (nm) spatial resolution. These measurements are done by scanning at least one small conductive tip over the device surface and recording at the same time topographical information and/or electrical signals.
Metal is preferably used as tip material due to its excellent electrical conductivity. Silicon (Si) tips with a thin metal coating, typically 10-25 nm thick, are commercially available since the early 1990""s but the metal coatings rapidly wear off. Therefore solid metal pyramids made by the so-called molding technique have been developed which are integrated into Si cantilevers or metal cantilevers. The metal cantilever probes, also called full metal probes, are preferred because they are highly conductive and require fewer process steps. In European Patent No 0899538xe2x80x9cA probe tip configuration, a method of fabricating probe tips and use thereofxe2x80x9d, (which is hereby incorporated by reference in its entirety) the inventors discloses a method of fabricating probe tips applying said so-called molding technique. The fabrication and use of a metal tip is disclosed in xe2x80x9cFabrication and use of metal tip and tip-on-tip probes for AFM-based device analysesxe2x80x9d, by T. Hantschel et al. Proc. of SPIE, 3512, pp. 92-103, 1998 (which is hereby incorporated by reference in its entirety). A method of manufacturing full metal probes is disclosed in xe2x80x9cFabrication of an all-metal atomic force microscope probexe2x80x9d, J.P. Rasmussen, Proc. Of Transducers ""97, p 463-466, 1997 or in xe2x80x9cThe fabrication of a full metal AFM probe and its applications for Si and InP device analysisxe2x80x9d, T. Hantschel et Al., Proc. Micromachining and Microfabrication conference, 1999 (both hereby Incorporated by reference in their entirety). This manufacturing method is schematically illustrated in FIG. 1e-1f. In FIGS. 1e and 1f the cantilever is drawn in side view for the purpose of teaching.
1. The wafer needs to be processed from both sides whereby precautions have to be taken to protect the non-processed side.
2. A lithographic tool, e.g. a mask aligner, is required to align patterns defined on the backside of the substrate to the front or topside of the wafer.
3. The backside of the silicon substrate will be etched to release at the front-side the cantilever with metal tip (see FIGS. 1a,e) and hence the substrate cannot be re-used.
4. This backside etching requires wafer scale uniformity of e.g. wafer thickness, etch speed, etch selectivity to avoid local over-etching of the tip and/or probe and it is a time consuming step.
5. During this etch step the tip, e.g. its radius or sidewalls, can be modified by the etch chemicals.
In European Patent No. 0763844 entitled xe2x80x9cmethod of manufacturing micro-tip and female mold substrate therefor, and method of manufacturing probe with micro-tipxe2x80x9d (which is hereby incorporated by reference in its entirety) some of these disadvantages are indicated. The inventor T. Yagi proposes a method to overcome some of the above-mentioned disadvantages, as illustrated in FIG. 2. According to European Patent No. 0763844 the first substrate is used to define the probe tip (23) while the second substrate is used to define the cantilever. Depending on the choice of the second substrate or its stack composition several types of cantilevers can be obtained. The first substrate could be re-used.
The above mentioned process method still suffers from the following disadvantages:
1. Two substrates are required to manufacture a probe tip with cantilever.
2. The metal, peeling layer and substrate have to be chosen such to obtain the correct relative strength of the different layers to allow tearing off the first second substrate from the probe tip.
3. The second substrate still requires an etching of the rear surface to define a cantilever and to release the probe tip. All problems related to such rear side etching remains.
Therefore, there is a need in the industry for a cost effective method of manufacturing tips and probes for use in scanning tunnelling or atomic force microscopes.
It is an aim of the invention to achieve a cost and time efficient method to manufacture probes used in scanning microscopy, e.g. AFM, STM, SSRM or likewise. This method comprises front side processing steps and avoids the backside lithography and time consuming etching steps required to create a probe membrane or to release the probe membrane. This method requires one substrate to form a full probe consisting of a probe tip, cantilever and probe membrane. This method allows easy removal of the probe prior to mounting the probe into a scanning microscope.
Another aim of the invention is to achieve a method of manufacturing a full metal probe comprising front side lithography steps, thereby allowing the probe tip and the cantilever to be separately processed. Hence the probe tip can be made from a material or metallisation scheme different from that of the cantilever. The characteristics of the probe tip can optimised independently from the cantilever.
It is one object of the invention to disclose a cost and time efficient method to manufacture probes used in scanning microscopy, e.g. AFM, STM, SSRM or likewise. This method comprises front side processing steps and avoids the backside lithography and time consuming etching steps required to create a probe membrane or to release the probe membrane.
One object of the invention is to a method comprising the steps of providing a substrate, depositing on said substrate a hard mask. During a first patterning step a mold is created in said hard mask and said substrate. A first layer is deposited on said patterned hard mask. This first layer has a sufficiently high enough adhesion to said hard mask to allow further processing, but this adhesion force is weak enough to be overcome when peeling the probe gradually from said underlying hard mask. During a second patterning step said first layer is patterned to form a probe tip configuration comprising at least a probe tip. By partially or fully under-etching said probe tip configuration from the front side of the substrate, the probe tip configuration can be removed from said substrate by lifting the probe.
Said method can further comprise the step of forming selective to said patterned first layer at least one additional layer before under-etching said patterned stack.
Said method can further comprise the step of mounting a holder chip to said probe tip configuration before peeling of the probe.
Said hard mask can be nitride, oxide, oxynitride or a combination of layers. Said first layer can be a metal, preferably a stack of Ti:W and Au.
Another object of the invention is to a method of manufacturing a full metal probe comprising front side lithography and processing steps, thereby allowing the probe tip and the cantilever to be separately processed. Hence the probe tip can be made from a material or metallisation scheme different from the cantilever. After depositing the first layer or a stack of layers the area of the probe tip is defined in an intermediate, third, patterning step. Hereafter a second layer can be deposited on said hard mask and said patterned first layer. This second layer will be patterned in the last patterning step to define the probe tip configuration consisting of probe membrane or contact area, cantilever and overlapping the probe tip area. Said second layer can be a metal, preferably a stack of Ti:W and Au.
Said method can further comprise the step of forming selective to said patterned second layer at least one additional layer before under-etching said patterned stack.